Forensic Science: Serology Unit: Blood Spatter Lab #3: Angle of Impact
Materials: Simulated blood, eyedropper, newspaper, clipboard, paper, yardstick, metric ruler
Question: How does the distance associated with the impact of blood to a surface affect the size of the
blood spatter?
Background: Forensic scientists examine bloodstain patterns to help determine the events surrounding a
crime scene involving blood spatter. The unique arrangement of the blood spatter can tell a story. Blood
spatter is just a collection of blood spots that may be different sizes and shapes formed by a variety of
methods.
Blood will spatter when an outside force breaks the surface tension of a blood drop. In other
words, blood will not spatter until it hits a surface. Blood spatter is typically in the shape of an ellipse (a
closed plane curve like an oval or circle). Mathematically speaking, the width and length of an ellipse
have a unique relationship. By figuring out the ratio of the two and using a trigonometric function,
scientists can determine the angle of impact at which the blood spatter occurred. This can help
investigators recreate a crime scene or confirm a suspect or victim's story. Look at the following example:
The width of the blood spatter above is 16 mm and the length is 24 mm. To determine the ratio between the
two, divide the width by the length. Convert the ratio to an angle measure using a table of trigonometric
ratios (see Table 1) or by using the sine function on a scientific calculator.
Steps to follow when using a trigonometric table:
1. Divide 16 by 24 (.6667).
2. Look at the chart and find the number .6667 under the Sine column.
3. The number falls between .6561 and .6691 on the table. It is closer to .6691.
4. .6691 corresponds to the angle measure 42.
5.
If you are going to calculate the angle using a calculator, follow the directions in your calculator
manual for the sine function.
6.
In conclusion, blood spatter that is 16 mm wide and 24 mm long hit the surface at
approximately a 42° angle.
Challenge: As ablood spatterexpert you have been called upon to examinepictures ofa crimescene in which a person was shot. Inthe
pictures theblood spatter is in theshape of almost perfect circles. Hereare a fewfacts from the accompanying policereport. Bob was shot by
Rob. Bob and Rob arethesameheight. Rob's testimonyclaims Bob attacked him. Rob fell to thefloor during the struggle. As Bob walked
toward him, Rob fired the gun in self-defense while he was still lying on the ground. Bob, having survived the shot, claims the two men
were standing face to face when Rob fired the gun. Based on the shape of the blood spatter, who is telling the truth? Justify your answer.
Follow the experiment procedures to help you determine who is telling the truth.
Procedure:
1. Create a data table in your notebook similar to the example below:
Sample
Number
Width (in mm)
Length (in mm)
Ratio
(as a Decimal)
Angle of Impact
in Degrees
2. Using a metric ruler, measure the length and width of a blood drop spatter appearing on the
Simulated Blood Drop Samples handout. Record the measurements in the appropriate
row on the Determining the Angle of Impact chart provided.
3. Divide the width by the length to get a decimal answer (round to the nearest tenthousandths) and record this in the appropriate ratio column.
4. Change this ratio into the corresponding angle measure by using the SINE function on a
scientific calculator or by locating the ratio on a trigonometric table on page 125 or one
provided by your teacher. Record this number in the appropriate column.
5. Repeat this procedure for the remaining blood spatter samples.
6. Look at the samples. Note the shapes of the blood spatter. The more circular the blood drop,
the closer the angle of impact is to 90°. The longer and skinnier the blood drop, the closer
to 10° it is.
7. Who is telling the truth, Bob or Rob? Did Rob fire the gun from a standing position or from lying on the floor? Be
sureto explain how youdetermined the angle at which the gun was fired when it hit Bob.